Date of Award

8-2012

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Roberto A. Lopez-Anido

Second Committee Member

Eric N. Landis

Third Committee Member

William G. Davids

Abstract

The objective of this thesis is to characterize the fatigue response and develop a crack onset prediction model for secondarily bonded woven fiber-reinforced polymer (FRP) composite T-joints under cyclic loading. Heavy woven E-glass fiber reinforced composites are widely used for marine and infrastructure applications. The manufacturing process commonly adopted by industry for these composites is vacuum assisted resin infusion. Typically marine construction requires secondarily bonded joints to create complex geometries. Secondarily bonded joints are manufactured by laying- up, infusing and curing a laminate directly onto a previously cured laminate without using an adhesive. This type of joint, typical in structural applications, is critical in failure analysis due to the high potential for crack initiation and propagation in the form of delamination. The secondarily bonded T-joint is one of the most common structural joints in marine construction and was chosen for this thesis. The heavy fiber tows, the weave pattern, the surface preparation, and the secondarily bonded manufacturing process introduce unique challenges in characterizing the fracture response of woven fabric composite materials. A fracture mechanics approach is adopted to characterize the material and develop the fatigue life prediction. A unified approach to interlaminar fracture toughness computation for Mode I, Mode II and three mixed-modes based on establishing a compliance calibration curve for the woven fiber composite material is presented. Mixed-mode fracture is a combination of Mode I and Mode II fracture. The proposed method does not require beam theory assumptions, making it more suitable for woven fiber composites. In this way the method offers a unified approach to establish interlaminar fracture toughness values of a highly variable material. The fatigue life of the material under Mode I, Mode II and three mixed-modes is characterized by establishing strain energy release rate versus number of cycles to delamination onset (G-N) curves. The approach is presented for Mode I fracture in ASTM D6115, however the standard is for unidirectional FRP composites; furthermore, there is not a published standard for Mode II or mixed-mode fracture. The G-N curves capture the variability of the woven material and the general distribution of cycles to crack onset. The G-N curves were compared to results from tests of solid and sandwich construction T-joints. Finite element (FE) models with the virtual crack closure technique (VCCT) were used to predict the fracture mode of the T-joints. The VCCT is used to characterize an interface crack within the context of linear-elastic fracture mechanics. The FE model with VCCT predicted Mode I behavior for the solid T-joints. A good correlation was with the Mode I G-N curve was observed. The FE model with VCCT predicted mixed-mode fracture with 90% Mode II fracture component for the sandwich T-joints; however sandwich T-joints showed better correlation with Mode II material coupon behavior. The method of predicting fatigue fracture behavior was observed to be reliable for the secondarily bonded woven FRP composite T-joint with both solid and sandwich constructions. The prediction method is beneficial for design and analysis of secondarily bonded joints as it reduces the time and effort required for component testing.

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